A null lens is commonly used in testing an optical surface (test surface). One example of its use is testing a large concave parabolic optical surface using an optical interferometer. It is convenient to test the optical surface using a diverging beam of light originating near the center of curvature of the surface. Unfortunately, the test surface does not return a well-corrected wavefront at this conjugate. Typically, large amounts of spherical aberration are present in the wavefront even when the test surface is of high quality.
The null lens is used to compensate for this wavefront error by providing a calibrated correction to the expected aberrations in the optical surface to be tested. A reflective Offner null is typically used in this case. (See, e.g., Optical Shop Testing, Second Edition, D. Malacara, ed., John Wiley & Sons, Inc., 1992, pp. 440.)
The Offner null requires two large low F-number spherical mirrors that are difficult to fabricate. It may also include a field lens, which makes it catadioptric—containing both mirrors and lenses. A field lens can introduce chromatic aberration, which results in extra wavefront distortion that changes with the wavelength of the light used to interrogate the surface. The Offner assembly is also relatively sensitive to tilt of the surface to be tested (hereafter referred to as field of view) and temperature variations of the assembly.
Because of the low F-number of the mirror surfaces included in the Offner assembly, an objective lens with a high numerical aperture is required to inject light into the test assembly. Such an objective lens is difficult to fabricate, calibrate, and align.
Many all-reflective (catoptric) null test assemblies require reflecting surfaces which are large relative to the size of the surface under test. These include full-aperture autocollimation flats (for testing concave paraboloidal mirrors from their focii, or testing complete telescope systems), and large spherical mirrors (for “Hindle” and “Ritchey Common” tests). In the autocollimation flat assembly, the flat mirror must be at least as big as the mirror under test. In assemblies using a single large concave spherical reflector to null the aberrated wavefront of a concave paraboloid, it is not possible to completely null the wavefront without making the reflector aspherical. The reflector also has to be about one-third the diameter of the mirror under test. If the mirror under test is, for example, a large primary telescope mirror exceeding 5 meters in diameter, the large mirrors required for the null assembly are difficult and expensive to fabricate and difficult to support mechanically. (See Malacara, Appendix 2.)
Embodiments of the present invention overcome these limitations by including a relatively small aspheric mirror in conjunction with a second small mirror to provide a nearly perfect nulling function. Additional optics provide proper imaging. The correction is contained on the aspheric mirror, making these additional optics relatively simple to fabricate and align. Sensitivity to figure errors in the aspheric mirror is reduced and the field of view of the null is increased compared to an Offner null designed for an equivalent test. Embodiments of the present invention for testing a mirror 6 meters in diameter have no mirrors with a diameter exceeding 0.3 meters included in the null assembly. These same embodiments employ a focus feed with an f-number no less than 3.3, as contrasted with the very fast speed (f˜1.25) needed for the autocollimation test configuration.